1. Field of the Invention
The presently disclosed subject matter relates to a piezoelectric actuator, and in particular to a structure of a piezoelectric actuator adapted to perform a translational motion in a direction of a thickness of a device (a z-direction movement), and a variable capacitor and an optical deflection device which include this structure.
2. Description of the Related Art
Actuators which perform a translational motion (a z-direction movement) in a direction of a thickness of a device can be used in various applications. For example, such an actuator can be used in one of two parallel flat-plate electrodes, so that the actuator can be used as a variable capacitor based upon change of an inter-electrode gap. Further, such an actuator can be combined with a reflecting mirror optical system, so that a device for performing optical path change of light entered in a mirror part can be obtained, and the device has various applications such that the device can be used as a pickup tracking actuator for reading an optical disk or a variable-focus scanner.
When applying the actuator to the above described device, electrostatic force drive, piezoelectric drive, and the like may be performed by the actuator. When the electrostatic force drive is performed, a voltage close to 100 V is required. On the other hand, when the piezoelectric drive is performed, a large displacement can be obtained by a relatively low voltage. In the piezoelectric drive, a piezoelectric unimorph cantilever which can obtain a large displacement with a simple structure is frequently used. The piezoelectric unimorph cantilever is configured such that a lower electrode, a piezoelectric body, and an upper electrode are stacked on an oscillation plate, and has a beam (cantilever) structure where one end portion of a lever part is fixed. By applying electric field to the piezoelectric body, the piezoelectric body is deformed such that the whole lever part is curved upward like a bow. Further, as an actuator which can obtain a larger displacement, a bimorph actuator obtained by stacking two piezoelectric body layers to sandwich an electrode and an oscillation plate is also used.
Japanese Patent Application Laid-Open No. 2004-127973, Japanese Patent Application Laid-Open No. 2008-005642, Japanese Patent Application Laid-Open No. 2010-251726, Park, J. Y. et al., “Micromachined RF MEMS tunable capacitors using piezoelectric actuators”, Microwave Symposium Digest, 2001 IEEE MTT-S International, vol. 3, pp. 2111-2114 (2001) and Kawakubo, T. et al., “RF-MEMS Tunable Capacitor With 3V Operation Using Folded Beam Piezoelectric Bimorph Actuator”, Journal of Microelectromechanical Systems, vol. 15, No. 6, pp. 1759-1765 (2006) disclose configuration examples of a variable capacitor utilizing a cantilever-type piezoelectric actuator. Yee, Y. et al., “PZT actuated micromirror for fine-tracking mechanism of high-density optical data storage”, Sensor and Actuator A89, pp. 166-173 (2001) discloses a configuration example of an optical pickup.